A material for fuel tanks of automobiles and motorcycles is required to have not only weldability but also resistance to general corrosion on its outer side (hereinafter called "cosmetic corrosion resistance") and to corrosion caused by fuels such as gasoline on its inner side (hereinafter called "fuel corrosion resistance"). Such corrosion resistance is collectively referred to as "corrosion resistance" or "corrosion resistance after forming". Conventionally, a ternesheet (10-25% Sn--Pb alloy-plated steel sheet) has widely been used as a material for fuel tanks. However, it has the following disadvantages: (i) Pb included in the ternesheet is harmful to the human body, (ii) the plated layer is easily dissolved in oxides of alcohols when an alcohol-containing fuel is used, and (iii) formation of pin holes in the plated layer is inevitable, resulting in preferential corrosion of iron from these pin holes since iron is electrochemically base compared with the plated layer, so perforation corrosion resistance is not satisfactory. An alternative to ternesheet, therefore, has long been sought.
Recently, in order to reduce the environmental problems caused by exhaust gases, an alcohol-containing fuel, called "gasohol", is being used increasingly in some countries. Gasohol is a mixture of gasoline and alcohol. For example, the mixture referred to as M15 contains about 15% methanol, and that referred to as M85 contains about 85% methanol. Conventional terneplate is easily corroded by such an alcoholic fuel, so a material which can exhibit improved resistance to corrosion caused by an alcohol-containing fuel is strongly desired.
For this purpose, it has been proposed to apply a Zn--Ni alloy electroplated steel sheet to fuel tanks because of its marked resistance to corrosion and its material cost. Prior art references in this respect are as follows.
Japanese Patent Application Laid-Open Specification No. 45396/1983 discloses a surface-treated steel sheet for fuel tanks having a Zn--Ni alloy plating with an Ni content of 5-50 wt % and a thickness of 0.5-20 .mu.m, and a chromate film on the Zn--Ni alloy plating.
Japanese Patent Application Laid-Open Specification No. 106058/1993 discloses a surface-treated steel sheet for fuel tanks having a Zn--Ni alloy plating with an Ni content of 8-20 wt % and a weight of 10-60 g/m.sup.2 and a chromate film on the plating.
These surface-treated steel sheets are excellent with respect to cosmetic corrosion resistance, but they are not adequate with respect to fuel corrosion resistance after they are formed into fuel tanks. Especially, fuel corrosion easily occurs under severe corrosive circumstances, e.g., when the plates are exposed to alcohol-containing fuels contaminated with salt water. However, if a chromate film or electroplated layer is thickened so as to further strengthen protection of the tank from fuel corrosion, weldability is inevitably degraded. Weldability is an essential characteristic for materials for fuel tanks.
From the viewpoint of providing cracks in a plating layer, the following prior art references are noted, but they are totally silent about corrosion resistance after forming.
Japanese Patent Application Laid-Open Specification No. 25679/1993 and No. 337099/1992 disclose surface-treated steel sheets with improved corrosion resistance, which exhibit an improvement in adhesion of coatings against impact, and which comprises a thin substrate layer of an Zn--Ni alloy plating having fine cracks with a width of 0.01-0.5 .mu.m, a crack density of 10-60% in terms of the total crack area, and a Zn--Ni alloy plating layer on the thin substrate Zn--Ni alloy layer. However, these surface-treated steel sheets are to be used for making outer panels of vehicles with improvement in impact adhesion, i.e., steel sheets used as outer panels of automobiles having a painting layer which does not swell even if the painting layer is impaired by bouncing of pebbles or by scratches. The impact adhesion of an upper plating layer of Zn--Ni alloy can be improved through the anchoring effect since the upper Zn--Ni alloy plating layer is placed into cracks of the plating underlayer.
Japanese Patent Application Laid-Open Specification No. 297490/1987 discloses a blackened, surface-treated steel sheet comprising a 0.5-2 .mu.m thick Zn--Ni alloy plating layer with a Ni content of 3-15%, and a 0.3-1.5 .mu.m thick Ni alloy plating layer with a Ni content of 15-75%, which is placed on the Zn--Ni alloy plating layer, fine cracks being formed uniformly over at least the surface of the Ni alloy plating layer.
An area of fine cracks 0.1-0.4 .mu.m wide, 1-10 .mu.m long, and 0.2-1 .mu.m deep comprises 60% or more of the total area of cracks. The presence of such fine cracks causes the steel sheet to be blackened. In addition, the above-mentioned steel sheet comprised of double plating layers has a Zn--Ni alloy plating layer with a low content of Ni, and a blackened layer to be placed thereon with a high content of Ni and having fine cracks. The adhesion of the blackened layer after forming is therefore improved markedly.
It is apparent that in the above-mentioned example, since the Ni content of the upper Zn--Ni plating layer is very large, a high level of corrosion resistance cannot be achieved even in the form of a plate if a chromate film is applied to the upper layer.
Furthermore, since the Zn--Ni plating alloy layer is of the dual layer type (thickness of the underlayer.gtoreq.thickness of the upper layer), and cracks formed in the upper layer of the plating do not propagate to the underlayer, cracks newly formed in the under layer during press forming expose the substrate steel sheet and the corrosion resistance after forming is degraded markedly.